Published European Patent 0 477 400 discloses a path transformer for a piezoelectric actuator in which the actuator transmits a lifting force to a master cylinder sealed by a cylinder support. Guided in this master cylinder is a slave piston, which likewise seals the master cylinder and thereby forms the hydraulic chamber. Arranged in the hydraulic chamber is a spring that presses the master cylinder and the slave piston apart. The slave piston mechanically transmits a lifting movement to a valve needle, for instance. When the actuator transmits a lifting movement to the master cylinder, this lifting movement is transmitted to the slave piston by the pressure of a hydraulic fluid in the hydraulic chamber, since the hydraulic fluid in the hydraulic chamber is not compressible and only a very small portion of the hydraulic fluid is able to escape through the ring gap during the short duration of a lift. In the rest phase, when the actuator does not exert any compressive force on the master cylinder, the spring pushes the slave piston out of the cylinder and, due to the generated vacuum pressure, the hydraulic fluid penetrates and replenishes the hydraulic chamber via the ring gap. In this manner, the path transformer automatically adapts to linear deformations and pressure-related expansions of a fuel injector.
One disadvantage of the coupler arrangement disclosed in the European Patent 0 477 400 is the high expense caused by the high manufacturing precision required for the components. Furthermore, in opening pulses that occur in close succession, the coupler medium escapes from the coupler gap and, due to the narrow width of the leakage gap, is unable to continue flowing fast enough, which means the switching dynamics of fuel injectors with hydraulic couplers is limited. German Patent No. 197 35 232 discloses the use of an electro-rheological liquid in a fuel injector, the fuel injector being provided with a damping element connected to the valve needle of the fuel injector to model the injection profile or the injected fuel quantity. In response to an excitation or de-excitation of the electromagnet, the damping element effects a flow of an electro-rheological fluid into a damping chamber via a capacitive component. With the aid of the capacitive component, the viscosity of the electro-rheological fluid is modifiable by an electronic control device as a function of operating parameters of the internal combustion engine, in such a way that the movement profile of the damping element is implemented such that the fuel spray-discharged via the spray-discharge orifice assumes a desired jet form or is spray-discharged at a desired time. The use of the rheological fluid for a compensation element for piezoelectric or magnetostrictive actuators is not described in German Patent No. 197 35 232, however.